Power converters for conversion of DC power to AC power--known as inverters--have long been known. More recently, SCR-type inverters have become very widely used, particularly such inverters having low, industrial frequency outputs (generally 60 Hz or 400 Hz), but having very high power ratings.
However, prior art inverter circuitry has provided a number of disadvantages, including particularly high distortion output, or high capital costs particularly because of the necessity to provide physically very large filter circuits. In addition, prior art inverters have been known to become unstable, particularly when subject to a dynamic load, i.e. one which may very rapidly as to its power requirements, or its power factor, or with sudden changes of the DC input voltage legal; and what has happened is that the AC output may undershoot or overshoot, or ringing may occur in the output or chopper circuit, following by hunting and feedback loop instability. In such cases, the output might collapse, the SCR's may misfire, and the inverter fails.
More especially, however, particularly with inverters of the sort which are particularly taught in applicant's U.S. Pat. No. 3,931,565, issued Jan. 6, 1976, is that a high stability has been achieved, and a low distortion output has been achieved--particularly when operated with Ferroresonant Voltage Regulating Circuits of the sort taught in applicant's U.S. Pat. No. 3,824,449 issued July 16, 1974 and applicant's U.S. Pat. No. 4,142,141 issued Feb. 27, 1979. Such inverter circuits as those previously taught by the applicant have, usually, 180 electrical degree commutation which effectively frees the inverter from effects of changes of power factor of the load, so that current and voltage stresses within the commutation circuits of the inverter are substantially constant and are predictable. However, it has been noted that, as with nearly all inverter circuits, greater energy efficiency would be desireable, and it has been determined that energy efficiency can be increased by the elimination of certain components used in prior art circuits, including those of the applicant.
It has, however, been noted that it is not enough merely to eliminate components such as by elimination of the de-coupling diodes as previously used, and as discussed in greater detail hereafter; additional circuit changes must also be made to the effect that voltage stresses on the components must be reduced as output currents increase to full load values; and such effects have unexpectedly been achieved by virtue of the elimination of the de-coupling diodes together with the provision of a non-DC-saturating chopper transformer--i.e., a transformer having an air gap in its core--and also the provision of a very low impedence commutating capacitor and a primary winding on the chopper transformer which is such as to suppress leakage flux. In general, the manner by which leakage flux is suppressed is by the provision of a bifilar primary winding on the chopper transformer; however, the provision of an interleaved winding would also achieve the same effect.
A further advantage that has been achieved by the provisions of circuits according to the present invention has been that, with a suitably dimensioned and rated air gap and primary winding on the chopper transformer, once commutation has started the commutation choke can be eliminated by short circuiting or otherwise, while still maintaining commutation of the chopper circuit.
By virtue of the elimination of the de-coupling diodes, a much better square-wave output of the chopper circuit may be achieved, thereby providing still greater energy efficiency and better sinusoidal wave forms from the output.
Because there is less voltage stress on the SCR's, as discussed hereafter, particularly on overload or short circuit conditions, an inverter according to the present invention demonstrates a greater or improved MTBF (mean time between failures) rating.
Other features of the present invention are the inclusion of input circuitry which limits inrush current and permits very fast start up of the inverter. Such "soft start" circuitry creates less stress on the DC power source, as well as stress on the components in the commutation circuits. Additionally, the circuits of the present invention provide very fast commutation, and the total commutation circuit setup is such as to provide quite sufficient head room in the event of instability in the power source, and particularly so as to provide better square-wave output.
Thus, the present invention comprises an inverter circuit, as described above, having the combination of at least:
(a) an input diode connected to the DC source for the inverter, and having its polarity arranged so as to be normally conductive; PA1 (b) an input filter choke in series with the DC input and the input diode; PA1 (c) an input, polarized capacitor across the DC input, with its positive side facing the positive side of the DC source; PA1 (d) a pair of SCR's facing in the same direction with respect to the DC input, and having a low impedence, unpolarized, AC commutation capacitor facing the SCR's on the side of each thereof which is remote from the DC source; PA1 (e) a drive circuit for the SCR's arranged so that, at any time, one or the other of the SCR's is conductive; PA1 (f) a transformer--which is essentially a DC chopper transformer--arranged with its primary winding connected so that at any instant of time at least a portion of the primary winding is in series with the SCR which is conductive at that instant; the AC output of the basic inverter circuitry being taken from the secondary winding of the transformer; PA1 (g) a commutation choke in series with the SCR's and connected in such a manner that each of the SCR's is in series with at least a portion of the commutation choke; PA1 (h) and a feedback circuit which comprises at least one diode arranged in counter-polarity to the polarized input capacitor and connected to the opposite polarity side of the polarized input capacitor, with the other side of the diode facing the primary winding of the transformer.
As stated, the transformer is such as to have either a bifilar or interleaved primary winding, and an air gap.
The above is a basic description of the principal arrangement of basic circuits according to the present invention. However, the DC chopper transformer may be a center-tapped transformer where each of the SCR's faces one or the other of the ends of the primary winding of the transformer, and the primary winding is center-tapped to one side of the DC input; or the DC chopper may be of the bridge variety having two pairs of SCR's where both SCR's of each pair face in the same direction and the pairs are oppositely faced with respect to the DC input, and where each of the SCR's in each pair faces one or the other of the ends of the primary winding of the transformer. In the latter circumstances--i.e., in the case of a bridge-type DC chopper--one or the other of each of the pairs of SCR's is conductive at any instant of time, in a manner so that both ends of the primary winding of the DC chopper transformer face a conductive SCR at all times.
Still further, while the discussion which follows is particularly related to a 180 electrical degree commutation, other types of commutation may be provided such as by the addition of an appropriately connected primary circuit for the transformer and additional SCR and firing circuits, so that a 120 electrical degree, 3-phase inverter is provided.